1. Field of the Invention
The present invention relates in general to screw pumps, and, more particularly, to improved screw pump rotors and methods of reducing slip flow in screw pumps.
2. Description of the Related Art
In the exploration for oil and gas the need to transport fluids (oil, water, gas, and foreign solids) from a wellhead to distant processing and/or storage facilities (instead of building new processing facilities near the wellheads) is well understood. Twin-screw pumps are increasingly being used to aid in the production of these wellhead fluids, resulting in increased production by lowering the pressure at the exit of the wellhead as well as a greater total recovery from the reservoir by allowing lower final reservoir pressures before abandoning production.
FIG. 1 illustrates a conventional twin-screw pump 10. This figure is presented simply to illustrate the main components of a twin-screw pump and should not be considered as limiting the invention disclosed herein in any way. As illustrated, the twin-screw pump 10 has two rotors 12 and 14 that are disposed within a close-fit casing or pump housing 16. Each rotor has a shaft 18A and 18B with one or more outwardly extending sets of screw threads 20 for at least a portion of the length of the shaft. The shafts 18A and 18B run axially within two overlapping cylindrical enclosures, collectively, a rotor enclosure, or liner, 19. The two rotors 12, 14 do not touch each other, but they have threads of opposed screws that are intertwined. Pump 10 will often be driven by a motor (not shown), which rotates rotors 12 and 14. A drive gear 22 on one of the shafts engages a second gear on the other shaft, such that, when the pump motor turns rotor 12, rotor 14 is turned at the same rate, but in an opposite direction. In operation, wellhead fluids, including particulate materials, are drawn into pump 10 at inlet 24. As the rotors 12 and 14 are turned, the threads 20, or more properly, rotor chambers 26 formed between adjacent threads 20 displace the wellhead fluids along the rotor shafts 18A and 18B towards an outlet chamber 28, which is the point of greatest pressure at the center of the rotors, from where the wellhead fluids are finally discharged from an outlet 30 of the pump 10. The rotor chambers 26 are not completely sealed, but under normal operating conditions the normal clearance spaces that exist between the rotors 12,14 and between each rotor and the rotor enclosure 19 are filled with transport fluid. The liquid portion of the transport fluid in these clearance spaces serves to limit the leakage of the pumped fluids between adjacent chambers. The quantity of fluid that does escape from the outlet side of the rotor back toward the inlet represents the pump slip flow, which is known to decrease the pump volumetric efficiency. As illustrated in FIG. 2 and just explained, pump slip flow (illustrated by the arrows in FIG. 2) can occur between each rotor and the rotor enclosure 19. As understood by those of ordinary skill in the art, other slip paths include slip between screw tip and adjacent rotor and between faces.
As understood by those of ordinary skill in the applicable arts, conventional twin-screw multiphase pumps face significant challenges. Consider for example, the following exemplary problems. First, assuming a fixed pressure rise per stage, as the total pressure rise requirement increases, the rotor length has to increase, resulting in an increased rotor deflection under the imposed pressure loading thereby creating a more eccentric alignment of the screws within the liner resulting in excessive slip between the screw rotor and the pump liner, if not contact and rubbing. Secondly, as the pump slip flow increases, sand particulates trapped in the slip flow leads to increased erosion/abrasion within the pump, particularly at the rotor tips by a phenomenon referred to as jetting. Such erosion/abrasion further leads to deterioration of the clearance profile and an increase in the pump slip flow. Finally, during periods of operation in which the transported fluids have a high gas-volume fraction, the temperature of the flow exiting the pump rises due to the heat generated during compression, leading to reduced clearances in the last pump stages due to variations in thermal expansion of the various pump parts, thereby possibly resulting in catastrophic seizure.
It would therefore be desirable to develop a pump rotor that will minimize or eliminate pump slip flow, resulting in a high differential pressure boost multiphase pump with a compact rotor length. In addition, better sealing between the edges of the rotor and the pump casing will also insure a reduction in solid particulate erosion/abrasion within clearances. Finally, having the ability to accommodate differences in thermal expansion as may occur when boosting high gas-volume fraction fluids may also reduce the likelihood of catastrophic seizures.